We are interested in the correct definition of a "gene" (see ...). Part of the confusion is due to popular science writers who don't get it right. For example, Richard Dawkins does some serious handwaving in The Selfish Gene and he compounds it in The Extended Phenotype.
Dawkins knows that his defintion of "gene" in the Selfish Gene is unusual so he returns to the subject in The Extended Phenotype in his discussion of the selfish replicator. Dawkins is forced to concede that his use of the word "gene" is incorrect. That's why he says,
I am happy to replace 'gene' with 'genetic replicator where there is any doubt.Nevertheless, he tries very hard to defend his point of view by claiming that geneticists and molecular biologists can't come up with a good definition of gene either. This leads him to make some very silly statements about genes and cistrons. He defines his genetic replicators in terms of alleles which means that they don't exist unless there is variation in the genome. He then goes on to restrict his discussion of changes in frequency to the results of natural selection, which means that his "genes" are effectively defined by the mechanism he prefers. This is why he quotes George Williams,
This is the rationale behind Williams's definition: 'In evolutionary theory, a gene could be defined as any hereditary information for which there is a favorable or unfavorable selection bias equal to several or many times its rate of endogenous change.'The hand-waving in The Selfish Gene is even more obvious,
....The Extended Phenotype p.89
In the title of this book the word gene means not a single cistron but something more subtle. My definition will not be to everyone's taste, but there is no universally agreed definition of a gene. Even if there were, there is nothing sacred about definitions. We can define a word how we like for our own purposes, provided we do so clearly and unambiguously. The definition I want to use comes from G.C. Williams. A gene is defined as any portion of chromosomal material that potentially lasts for enough generations to serve as a unit of natural selection.In the new version of The Selfish Gene (1989) Dawkins adds a footnote where he again addresses his critics, especially Sewall Wright. Dawkins defends his definition of a gene as a unit of selection.
....The Selfish Gene p.28
I am using the word gene to mean a genetic unit that is small enough to last for a number of generations and to be distributed around in many copies. ... The more likely a length of chromosome is to be split by crossing-over, or altered by mutations of various kinds, the less it qualifies to be called a gene in the sense I am using the term.
....The Selfish Gene (1989) p.32
I said that I preferred to think of the gene as the fundamental unit of natural selection, and therefore the fundamental unit of self-interest. What I have now done is to *define* [Dawkins' emphasis] the gene in such a way that I cannot really help being right!The fact that Dawkins uses the word "gene" in such a non-standard way is not an issue as long as one recognizes that the Dawkins "gene" has nothing to do with the genes that molecular biologists and geneticists talk about. It's not an issue as long as one doesn't try and pretend that Dawkins has avoided handwaving and "clearly" refuted the problems raised by his critics.
....The Selfish Gene (1989) p.32
The most reasonable definition of gene is that it is a piece of DNA that is transcribed but there are exceptions to everything in biology. Some genes are made of RNA, for example, and sometimes it's better to define a gene in terms of the protein it encodes. In no case is it reasonable to define a gene in terms of its ability to be selected or whether recombination can occur within it.